The present invention relates generally to a method for evaluating fluids and, more specifically, to a technique employing a single electrode for conductivity analysis, including on-line analysis, of fluids subject to degradation and/or contamination.
Fluids, such as oils, lubricants, cooking oils and other fluids are used in ways that cause their degradation. For example, it is common to lubricate and cool the components of operating equipment by wetting them with an oil or lubricant. As the oil or lubricant carries out these functions, it experiences environmental stresses which cause it to degrade. As another example, oils used as transmission fluids and in hydraulic systems are subjected to stresses such as pressure, frequent movement and heat. As still another example, cooking oils undergo severe thermal-oxidative stresses. The degradation of the base stock can lead to the production of acids within the cooking oil which affect the taste of the food.
Because of this degradation, antioxidants are frequently added to fluids to reduce their rate of degradation. As long as the antioxidant system remains intact, the oxidative degradation of the base stock is minimal, and so are changes in the properties of the fluid. However, the antioxidants in the fluid gradually deplete over time. Eventually, the antioxidants become ineffective allowing changes in the physical properties of the base stock to occur. At that point, the fluid is no longer able to protect the equipment, and its useful life is completed. The continued use of a fluid after its useful life can result in excessive component wear and eventual equipment failure of the equipment in which the fluid is used. In addition, abnormal depletion rates for antioxidants may indicate accelerated fluid oxidation leading to severe wear problems prior to equipment failure.
Not all fluids contain antioxidants. In this case, the degradation of the base stock can lead to the production of acids within the fluid which render it useless. For example, cooking oils become rancid when they degrade past a certain point.
Early detection of liquid contamination is also important, as is early detection of failures in cooking oils. If a liquid from another part of a system leaks into the fluid, the fluid could become useless because of that contamination. For example, if a coolant leaks into the lubrication system of an engine, the lubricating fluid degrades faster than uncontaminated fluid and becomes useless in that engine. Use of a contaminated lubricating fluid can damage the engine if the fluid is not replaced.
Since it is undesirable to use a fluid beyond its useful life, an equipment operator will establish scheduled fluid changes for the equipment. The length of operating time between scheduled changes is chosen conservatively so that a fluid, which is beyond its useful life, does not remain in the equipment and damage the equipment. Unfortunately, this conservative approach results in fluids which still have useful lives being discarded.
The ability to analyze fluids, such as oils, lubricants, cooking oils and other fluids, for antioxidant depletion, oxidation initiator buildup, product buildup and liquid contamination would eliminate the need to perform fluid changes on the basis of a fixed schedule. This would allow longer use of the fluid providing savings in material and labor costs.
U.S. Pat. Nos. 4,744,870 and 4,764,258 to Kauffman, which are both assigned to the assignee of the present invention, disclose methods for determining the remaining useful life of fluids. These methods are fast, accurate, easy to operate and can be performed with inexpensive equipment. In these methods, fluid samples are mixed with a solvent and an electrolyte in either an organic base or a solid substrate, depending on the type of fluid which is being tested. A sample of fluid is removed from the system, placed in an electrolytic cell and subjected to a cyclic voltammetric analysis. The current generated during the cyclic voltammetric analysis is measured and recorded. The remaining useful life of the fluid is then determined from the oxidation or reduction wave height. However, these methods can only be performed off-line and are limited to fluids containing antioxidants.
U.S. Pat. No. 5,071,527, also to Kauffman and assigned to assignee of the present invention, solves the problem cited above by providing a more complete fluid analysis, including on-line analysis, which measures antioxidant depletion, oxidation initiator buildup, product buildup and liquid contamination. The method uses a triangular wave form which is applied to a microelectrode. The resulting current is monitored at a second microelectrode. In high resistance fluids, such as oils, fuels, etc., a third electrode is used as a reference voltage for the applied voltage wave form. This technique uses a voltage range of about .+-.120V to cause electro-oxidation of antioxidants and electro-reduction of hydroperoxides and other thermal-oxidation products. The current flow produced by the electro-chemical reactions is then used to monitor the remaining useful life of the monitored fluid.
While the technique taught in U.S. Pat. No. 5,071,527 provides a useful and complete fluid analysis, often such an analysis is too extensive and the equipment necessary to conduct such an analysis is expensive. Thus, a need exists for a fluid analysis technique which requires less circuitry and is less expensive, even though it might not be as complete.